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   难度：Medium
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   <h1 class="question_title">
    221. Maximal Square
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   <p>
    Given a 2D binary matrix filled with 0's and 1's, find the largest square containing only 1's and return its area.
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   <p>
    <strong>
     Example:
    </strong>
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   <pre>
<strong>Input: 
</strong>
1 0 1 0 0
1 0 <font color="red">1</font> <font color="red">1</font> 1
1 1 <font color="red">1</font> <font color="red">1</font> 1
1 0 0 1 0

<strong>Output: </strong>4
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   <h1 class="question_title">
    221. 最大正方形
   </h1>
   <p>
    在一个由 0 和 1 组成的二维矩阵内，找到只包含 1 的最大正方形，并返回其面积。
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   <p>
    <strong>
     示例:
    </strong>
   </p>
   <pre><strong>输入: 
</strong>
1 0 1 0 0
1 0 <strong>1 1</strong> 1
1 1 <strong>1 1 </strong>1
1 0 0 1 0

<strong>输出: </strong>4</pre>
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